Pathogenic microorganisms are known that do not raise a protective enteric immune response in mammals (non-enteric pathogens). Up to now it has been believed that protection against infection by such non-enteric pathogens could not be obtained by oral immunization, especially when antigens from the pathogens were used as opposed to complete live or attenuated pathogens.
Such pathogenic microorganisms usually invade by non-enteric routes, especially through punctures, abrasions, cuts or other breaches in the skin, e.g. through blood transfusion.
Examples of diseases caused by non-enteric pathogens are: hepatitis B, hepatitis C, hepatitis delta, yellow fever, Lassa fever, dengue hemoragic fever, rabies, tetanus, staphylococcus aureous infections, yaws, relapsing fever, rat bite fever, bubonic plague, typhoid fever and spotted fever.
As an example of the above, hepatitis B virus (HBV) is responsible for significant morbidity and mortality in spite of the availability of efficacious parenteral vaccines. In 1996 it was estimated that some 115 million people were infected with HBV. Mortality caused by this disease is estimated to be 1 million cases per year. In developed countries such as the US, immunization rates for HBV remain below targeted objectives and there are over 300,000 new cases annually and 5,000 deaths each year as a result of HBV infection. In addition, a review of the prevalence of HBV infection in the US between 1976 and 1994 indicated that there was no significant decrease in HBV infection during that period, despite the availability of hepatitis B vaccine. Thus, in developed countries there is a need to improve the availability of and access to effective alternatives to current parenteral vaccines. This is even more important as the number of vaccines that are becoming part of childhood immunizations increases since there are practical considerations in how to safely and effectively administer the multiple antigens that are becoming part of the pediatric immunization schedule.
Another concern is that a significant proportion of the global morbidity and mortality is localized in developing countries where HBV is endemic. As an example, in rural Malawi evidence of HBV infection was found in 72% of women delivering in hospital and chronic carriage was 13%. In these settings, current parenteral vaccines are of limited availability because of the need for cold storage and the significant cost of the vaccines. While significant initiatives have begun to address the issue of how to provide hepatitis B vaccines to developing countries, alternative approaches are needed. Although immunization rates in developed countries may be on the increase, in the absence of an effective global immunization program for hepatitis B, there will continue to be importation of hepatitis B disease into developed countries from developing countries.
An alternative to parenteral immunizations for a few diseases are vaccines that can be delivered orally. As previously discussed, oral vaccines are generally not effective against non-enteric pathogens.
A specific approach to oral immunization has been proposed by expressing antigens in transgenic plant tissue followed by ingestion. This technique, in one step might have the potential to provide both a less complex manufacturing process and to provide the antigen in a “matrix” that would be suitable for oral immunization. In addition, plant tissues, such as potato tubers, have a distinct advantage in that vegetables, even in the raw state, have a long history of safety in the marketplace. Lastly, transgenic plant tissue expressing antigens that are delivered orally may have the added advantage that both humoral and mucosal immunity could be stimulated, resulting in the potential to protect mucosal surfaces more effectively than parenteral immunization alone might accomplish. Up to now, it has not been found that transgenic plant tissue expressing non-enteric pathogen antigenic material would be any more effective as an oral vaccine than direct oral intake of the purified antigen.
Plants expressing hepatitis B surface antigen (HBsAg) have in fact been developed but have also disappointingly been found to create little or unacceptably low immune responses in animals ingesting them even though HBsAg isolated from plants have been found to raise an immune response when administered parenterally. As used throughout this specification, HBsAg is intended to be exemplary and to represent other non-enteric pathogen antigens known to raise an immune response when parenterally administered.